Cat blood is broken down into three different blood types; A, B and AB. Problems occur when an A type tom is bred to a B type queen. All of the kittens will be type A because A is the dominant gene and B is the recessive. When they nurse, the kittens receive antibodies from the queen through the colostrum. The queen’s antibodies will attach to the kitten’s red blood cells, leading to their ultimate destruction. Clinically, some of the kittens will die within hours of ingesting the colostrum while others might fail to thrive over several days leading to the term “kitten fading syndrome”.
Once this immune reaction starts, it is difficult to control and save the kitten’s life. That’s why I strongly recommend blood typing all cats before breeding. Even though some breeds such as Siamese, Burmese, American Shorthair, Oriental Shorthair, Russian Blue and Tonkinese have a low incidence of type B, I still think it is a good idea to test as mutations may occur. Blood type B is most prevalent in Devon Rex, Scottish Folds, British Shorthair, Exotic Shorthair and Cornish Rex. Birmans are sometimes included in this group although I have not observed this clinically.
What should you do if your cat is already bred and then you find out that the blood types between the queen and tom are incompatible? If this situation occurs, do not let the kittens nurse from their mother during the first 24 hours. Give them milk replacer or better yet, find a type B lactating queen and place them with her. After the first 24 hours, gut closure occurs which means the kittens can no longer absorb antibodies through their digestive tracts. At this point, they may be returned to the queen.
Reference:
Little, S., Feline blood Types and Neonatal Isoerythrolysis, Winn Feline Foundation.